It is common that new mobile phones and smart phones have the function of playback of digital audio data, and a headphone is often connected to listen to the sound. In the case of playback of audio sound through a headphone, an earphone speaker, LINE output, or other channels, the charge pump type of DC-DC converter is used in the power supply circuit for the audio output unit, since the output wattage of the audio signal also can be small. Since an inductor for an external part is not required when a charge pump type of DC-DC converter is used, the charge pump type is suitable for small electronic devices for audio playback with a headphone or other means.
However, when the charge pump type of DC-DC converter is used, the spurious element of the constant frequency is contained in the output ripple voltage when the load current is constant, since that intermittent operation is executed for increasing the voltage only when the output voltage falls below the reference voltage.
The spurious element is generated in the audible bandwidth and affects digital-to-analog converter (DAC) output and the output amplifier that utilizes the DC-DC converter as a power supply, depending on the load current and the drive capability of the DC-DC converter. At this time, if the power supply rejection ratio (PSRR) of the circuits such as DAC and the output amplifiers is insufficient, the spurious element of the audible bandwidth mixes with the output as provided by the output amplifier, the undesirable noise becomes audible, and the S/N ratio also deteriorates. In particular, single frequency sound is present in the output of the output amplifier in the form of noise when there is no signal, which might cause unpleasantness for the listener.
In order to reduce the spurious noise element, constraining the current by increasing ON resistance of the switching MOS for switching the connection of the flying capacitor and the decoupling capacitor within the charge pump type of DC-DC converter has been considered. When increasing ON resistance of the MOS for switching, the loss caused by heat would increase, and the voltage conversion efficiency would decrease.